This invention relates generally to electrical propulsion systems for diesel electric locomotives equipped with alternating current traction motors and, more particularly, to a method and apparatus for enabling continued operation of the locomotive in the event of a failure of a current sensor.
In a conventional diesel electric locomotive, a thermal prime mover (typically a 16 cylinder turbo-charged diesel engine) is used to drive an electrical transmission comprising a synchronous generator that supplies electric current to a plurality of alternating current (AC) traction motors whose rotors are drivingly coupled through speed reducing gearing to the respective axle wheel sets of the locomotive. The generator typically comprises a main three-phase traction alternator, the rotor of which is mechanically coupled to the output shaft of the diesel engine. When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in three-phase armature windings on the stator of the alternator. These voltages are rectified to produce a controlled amplitude DC voltage and then applied to one or more inverters which control the effective frequency of alternating current to be supplied to the field windings of the AC traction motors. The effective AC excitation frequency produced by the inverters controls the speed of the AC motors with power being controlled by pulse width modulation of the AC waveform.
In normal motoring operation, the propulsion system of the diesel electric locomotive is so controlled as to establish a balanced steady state condition wherein the engine driven alternator produces, for each discrete position of a throttle handle, a substantially constant optimum amount of electrical power for the traction motors. In practice, suitable means are provided for overriding normal operations of the propulsion controls and reducing engine load in response to certain abnormal conditions, such as loss of wheel adhesion or a load exceeding the power capability of the engine at whatever engine speed the throttle is commanding. This response, generally referred to as deration, reduces traction power, thereby helping the locomotive recover from such temporary conditions and/or preventing serious damage to the engine.
In addition, the propulsion control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Typically, both the output voltage of the rectifier connected to the output of the alternator and the output current from the rectifier are monitored with appropriate sensors which provide both a feedback control of the propulsion system operation and also serve to prevent over-voltage and/or over-current conditions. At low locomotive speeds, the traction motor armatures are rotating slowly so that their back EMF is low. A low alternator voltage can now produce a maximum motor current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant and this magnitude level whenever locomotive speed is high since the traction motor armatures are rotating rapidly and have a high back EMF and the alternator voltage must be high to produce the required load current.
In conventional direct current (DC) traction motor powered locomotives, it is conventional practice to shut down the propulsion system upon detection of an abnormal current indication from the current sensor. Such practice is common since DC motors are susceptible to various high current conditions such as a flashover at the motor commutator or arcing between brushes on the commutator. Many different systems are disclosed in the prior art for automatically detecting and recovering from such flashover conditions. The significant feature of the DC motor is that it will recover from a flashover condition and thereby continue to be operable so long as the flashover condition is detected and extinguished prior to substantial damage being done to the commutator or brushes. In contrast, the more recently introduced AC traction motor powered locomotives do not have commutators which are subject to flashover conditions. In the event that an AC motor experiences a short circuit condition, that motor is irreparably damaged and cannot be brought back into service without stopping the locomotive and removing and overhauling the motor. Accordingly, it is common practice in an AC locomotive to shut down the locomotive when an abnormal current condition is detected. In many such systems, the abnormal current condition is caused by a failure of the current sensor rather than a failure of the AC electric motor. Accordingly, it is desirable to provide a propulsion system which includes means for detecting failure of the current sensor and for enabling continued operation of the locomotive in the event of a current sensor failure.